Conventionally, martensitic stainless steel has been widely used in oil-well environments. A conventional oil-well environment contains carbon dioxide gas (CO2) and/or chloride ions (Cl−). A martensitic stainless steel containing about 13 mass % Cr (hereinafter referred to as 13% Cr steel) has good corrosion resistance in such a conventional oil-well environment.
In recent years, higher oil prices have prompted development of deep-sea oil wells. Deep-sea oil wells are located at large depths. In addition, deep-sea oil wells have high corrosivity and high temperatures. More specifically, a deep-sea oil well contains high-temperature corrosive gases. Such corrosive gases contain CO2 and/or Cl−, and may contain hydrogen sulfide gas. A corrosion reaction at a high temperature is severer than a corrosion reaction at room temperature. In view of this, an oil-well steel for use in a deep-sea oil well is required to have a strength and corrosion resistance higher than those of a 13% Cr steel.
A duplex stainless steel has a higher Cr content than a 13% Cr steel. Thus, a duplex stainless steel has a higher corrosion resistance than a 13% Cr steel. A duplex stainless steel may be, for example, a 22% Cr steel containing 22% Cr, or a 25% Cr steel containing 25% Cr. However, a duplex stainless steel is expensive as it contains a larger amount of alloy elements. Thus, there is a demand for a stainless steel that has a higher corrosion resistance than a 13% Cr steel and is less expensive than a duplex stainless steel.
To address this demand, a stainless steel containing 15.5 to 18% Cr and having high corrosion resistance in high-temperature oil-well environments has been proposed. JP 2005-336595 A (Patent Document 1) proposes a stainless steel pipe having high strength and having carbon dioxide gas corrosion resistance in high-temperature environments at 230° C. The chemical composition of this steel pipe includes 15.5 to 18% Cr, 1.5 to 5% Ni, and 1 to 3.5% Mo, satisfies Cr+0.65Ni+0.6Mo+0.55Cu−20C≥19.5 and satisfies Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≥11.5. The metal structure of this steel pipe contains 10 to 60% ferrite and 30% or less austenite, the balance being martensite.
WO 2010/050519 A (Patent Document 2) proposes a stainless steel pipe having corrosion resistance in high-temperature carbon dioxide gas environments at 200° C. and having high sulfide stress corrosion cracking resistance even when the environment temperature in the oil well or gas well falls after removal of oil or gas is temporarily stopped. The chemical composition of this steel pipe includes more than 16% to 18% Cr, more than 2% to 3% Mo, 1 to 3.5% Cu and 3 to less than 5% Ni, and satisfies [Mn]×([N]−0.0045)≤0.001. The metal structure of this steel pipe contains, by volume ratio, 10 to 40% ferrite and 10% or less retained austenite, the balance being martensite.
WO 2010/134498 (Patent Document 3) proposes a high-strength stainless steel having good corrosion resistance in high-temperature environments and having good SSC resistance at room temperature. The chemical composition of this steel includes more than 16% to 18% Cr, 1.6 to 4.0% Mo, 1.5 to 3.0 Cu and more than 4.0 to 5.6% Ni, satisfies Cr+Cu+Ni+Mo≥25.5, and satisfies −8≤30(C+N)+0.5Mn+Ni+Cu/2+8.2−1.1(Cr+Mo)≤−4. The metal structure of this steel contains martensite, 10 to 40% ferrite, and retained austenite, where the ferrite distribution ratio is higher than 85%.
In high Cr stainless steels containing 15.5 to 18% Cr disclosed in these documents, the low-temperature toughness may often be insufficient. JP 2010-209402 A (Patent Document 4) proposes a high-strength stainless steel pipe for an oil well with good low-temperature toughness. This steel pipe contains 15.5 to 17.5% Cr, where the distance between any two points in the largest crystal grain in the microstructure is not higher than 200 μm (in other words, the crystal grain diameter is not larger than 200 μm). Further, WO 2013/179667 (Patent Document 5) describes that a steel has both good corrosion resistance and good low-temperature toughness if it has a microstructure in which the GSI value, which is defined as the number of ferrite-martensite grain boundaries present per unit length along a line segment extending in the wall-thickness direction.